The structure of the glycan of human thrombin has been determined, and the functional significance of this moiety has been sought. Enzymatic removal of the terminal mono-, di- and trisaccharides of the carbohydrate chain does not alter thrombin's ability to clot fibrinogen, to bind and stimulate platelets or to bind to fibrin. However, modification of the glycan may enhance thrombin's interaction with anti-thrombin III. Determining the functional significance of the thrombin glycan may lead to improved understanding of thrombotic disorders.